Corrosion inhibition



United States Patent CORROSION INHIBITION Lewis J. Brown, North Wales,Pa., and Samuel Spring,

Van Nuys, Califi, assignors to .Pennsalt Chemicals Cor- This inventionrelates to the corrosion protection of metal surfaces, and moreparticularly to the application of corrosion-inhibiting films tosurfaces composed of zinc, cadmium, iron, steel, aluminum, etc.

:Users of metal articles will, for their various and particularpurposes, apply different types of protective finishes to the metal whenthey wish to provide it with relatively permanent resistance tocorrosion. There still remains the problem of supplying such articles tothe ulti mate user in fresh and uncorroded condition, even though theymight have to be' handled, shipped and stored in the interim periodbetween fabrication and delivery. Some kind of interim protection mustbe provided for this period, since practically all metals are quicklyattacked when exposed to even ordinary atmospheric conditions.

Manytypes of treatment to provide this protection have been proposed, asfor example the process described in US. Patent 2,333,206, but ingeneral these are all unsatisfactory for at least one of the reasons,that they provide protection at the sacrifice of appearance, i.e., thatthey dull, discolor, streak or stain the metal surface; that they dontprovide adequate corrosion protection, particularly under humid oractually wet conditions; or that they are too expensive. Of thesedrawbacks, the loss of good appearance is from the-practical standpointby'far the most serious, since it has a controlling effect on themarketability of the metal article, and customer appeal is greatlyreduced in articles which dont appear to have a fresh, lustrous metallicsurface. Appearance is equated in the customers mind with essentialquality, and this conclusion is more often than not justified when thedamage to appearance is of a permanent nature. The conventional chromateand phosphate conversion coatings, though usually affording adequatecorrosion protection, have this drawback, that they permanently dull orstain the metal surface.

Accordingly, the primary object of the present invention is to providemetal surfaces with a corrosion-inhibiting film which does not impair orin any way alter the metallic appearance of the surface. Another obiectis to provide metal surfaces with a coating which will protect thesurface adequately from the corrosive conditions that may be encounteredduring the period between fabrication and ultimate use. A further objectis to provide such interim protection at a reasonable cost.

These and other objects have been achieved by applying to asubstantially clean metal surface two suecessive coating solutions, thefirst of whichcontains a hydrophobic film-forming material, hereinafterdefined,

and the second containing hexavalent chromium. The

order in which these solutions are applied is critical,

2,918,390 Ce .Palt,ented Dec. 22, 1959,

in that corrosion resistance of the resulting film is practicallynon-existant when the order given above is reversed. The solutionsemployed are extremely dilute, and deposit on the metal a vanishinglythin film, believed to approach a monomolecular film in thickness, whichnonetheless is su'ificiently'aclh'erent, water-repellent and continuousto confer a high degree of corrosion resistance'to the metal surface.The most significant'feature of this film, however, is that it in no waychanges the appearance of the metal: it is essentially colorless, clear,transparent and for all practical purposes invisible, so that the metalappears through it as fresh and lustrous as before treatment.

With regard to the hydrophobic film-forming material hereinabovereferred to, this consists of a salt of a fatty acid amine; moreprecisely, it has the general structure wherein R is an aliphatichydrocarbon group containing at least 8 carbon atoms and X is an anion,preferably of the group consisting of acetate, lactate, and chloride.Examples of typical compounds having this structure include octylamineacetate, dodecylamine acetate, octadecenylamine acetate, octadecylaminelactate, decylarnine chloride, and the like. Preferably the aliphatichydrocarbon group is an alkyl group of from 8 to 18 carbon atoms(corresponding roughly to the length obtainable from the common fattyacid residues). The acetate is the anion of choice, being the leastexpensive and most readily available. A particularly good compoundforlthe purpose of this invention is a commercial product of Armour andCompany, going by the trade name Armac HT, which is a hydrogenatedtallow amine acetate, i.e., a mixture comprising primarily hexadecyl andoctadecylamine acetates with a small amount of octadecenylamine acetatealso present.

In practicing the present invention, the metal should, to begin with, besubstantially clean; that is, it should be free from gross contaminationby oil, shop dirt, rust, etc. In general, the cleanerthemetal is priorto treatment, the more effective the treatment will be. Conventionalcleaning methods, 'such as alkaline spray or immersion cleaninvapor'degreasing, etc. may be used if needed. The metal may be passeddirectly from the cleaning operation, if any, to the treatmentwith thefatty amine salt without intermediate drying, although it is immaterialwhether the work piece is wet or dry. It is only advantageous that thesurface be free from contamination, such as entrained alkali from thecleaner, so that the amine salt solution is not contaminated. The aminesalts defined above are all either soluble or dispersible in water, andin practicing this invention they are applied to the metal in the formof a very dilute aqueous solution. The concentration of the amine saltin the treating solution should be about 0.01 to 1.0% by weight, andpreferably about 0.05 to 0.20%. The solution is applied to the metalmost conveniently by immersion for a few seconds, followed by a shortdraining period, but other conventional methods of application such asspraying or brushing may also be used. Longer periods of immersion, etc.are not harmful; the temperature of the solution is not critical.

Following treatment with the amine salt, the metal may be immediatelypassed to the solution containing hexavalent chromium withoutintermediate drying, though it is preferred to rinse the surface first,again to minimize drag-out and contamination of the chromium solution.The surface may be dried, but this is quite unnecessary and only adds anextra time-consuming step. For the chromium solution, compounds ofhexavalent chromium which possess some water solubility are used, forexample chromic acid or sodium or potassium dichromate, etc. Theconcentration should be within the range of 0.05 to 0.5% by weight(calculated as CrO and preferably between about 0.1 and 0.2%. Again,simple immersion in the solution is the easiest method of application,though other methods may be employed. The duration of application andthe solution temperature are not critical.

After application of the two solutions, in the order given, the surfacemay be force dried or simply permitted to air dry. The metal now has afilm on it that has a weight of less than 5 milligrams per square foot,and probably on the order of 1 to 3 mg./sq. ft. This film is visuallyimperceptible, yet without any further treatment, the metal is equippedto withstand the rigors of handling and storage, even when the latter iswithout benefit of protection from the elements.

It has further been discovered that the remarkable resistance tocorrosion conferred by this extremely thin film is surprisingly evenfurther enhanced when the metal is given a pretreatment prior toapplication of the amine salt and chromium solutions, this pretreatmentconsisting of cathodic cleaning in an alkaline solution. The reason forthe increased corrosion resistance resulting from this added processstep is still obscure, but the practical advantages are unequivocal.

Accordingly, in the preferred practice of the present invention, themetal surface, even though it may be quite clean and fresh, as forexample metal that is just freshly machined or galvanized, etc., isimmersed in an alkaline cleaner in an electrolytic cleaning bath whereinthe metal to be cleaned is made the cathode. With regard to thiscleaning step, it follows the conventional practice for this type ofcleaning. The solution may be any of the usual alkalies, such as causticsoda, alkali silicates, phosphates, carbonates, etc., and may containother conventional cleaning additives such as sequestrants orsurfactants. The strength of the cleaner can vary within wide limits, ascan its temperaature and total alkalinity. The time of immersion can bea few seconds or a few minutes, as described, and the current densitywill vary with the type of metal being treated; for example a current of10 to 5 amperes per square foot may be used for zinc surfaces, while upto 100 amps/sq. ft. may be appropriate for steel. The metal surfaceshould be carefully rinsed following this treatment and prior to theamine salt and chromate treatments. Again, the order of steps of thepreferred process is critical, and should be followed as given herein toprocure adequate corrosion protection.

-A further preferred aspect of the present invention is that it has beenfound advantageous to prepare both the fatty amine salt solution and thechromium solution with water which is substantially free from a certaingroup of anions which either tend to initiate corrosive processes orwhich form insoluble fatty amine salts, notably chlorides, sulfates,phosphates and silicates. Any rinse water that may be employed in theprocess should also be preferably free of these contaminants. The sourceof water to be used, if contaminated with such anions, can be treatedwith anion exchange media, or completely deionized water can be used,or, if a convenient source is present, it may be expedient to usedistilled water or steam condensate. In general, the undesired anionsshould not be present in amounts in excess of about parts per millionfor best results, though the use of reasonably s'oftwater may besufficient protec- -ftic an against their harmful effects in manyapplications. Where the cost of supplying suitable water must beconsidered, its use may be restricted just to the make-up of thechromium bath, where the absence particularly of chlorides and sulfatesis most beneficial, since these anions tend either to etch the metal orto deposit unsightly stains, or both.

The following examples will help to illustrate the practice of thisinvention and the practical advantages derived therefrom; it should beunderstood that their purpose is only illustrative and not limiting.

EXAMPLE I Test panels cut from commercial untreated galvanized steelwere cathodically electrocleaned in a 2 oz./gal. solution of aproprietary alkaline cleaner, rinsed, and then immersed for 15 secondsin a distilled water solution of Armac HT, the solution having aconcentration of 1 gram per liter (0.1%) of the amine salt and atemperature of 60 C. They were then immersed for 15 seconds in adistilled water solution of chromic acid, the solution having aconcentration of 1 g./l. (0.1%) and a temperature of 70 C., and airblast dried to remove the remaining beads of water. The resultingsurfaces had an unchanged, metallic appearance and showed stronghydrophobicity.

After treatment, these panels were subjected to a battery of corrosiontests, designed to simulate severely corrosive storage conditions, theprocedures of which are as follows:

Test 1 Water film.Panels are stacked with a few milliliters of waterplaced between each panel; the stacks are clamped and held for 24 hoursat room temperature EXAMPLE II Galvanized steel panels were cleaned for15 seconds at 60 C. in a 2 oz./ gal. alkaline cleaner. These panels weredivided into two groups; the first group was simply immersed in thecleaner, while the second group were cathodically cleaned in the samecleaner at 25 amps/sq. ft. After cleaning all the panels were rinsed andtreated with the amine salt and chromic acid solutions in the samemanner as described in Example I.

EXAMPLE III EXAMPLE IV Panels of 52 S aluminum alloy were cleaned byimmersion in an alkaline cleaner for 15 seconds at a temperature of C.They were then immersed for 15 seconds in a l g./l. solution of Armac HTin distilled water at 60 C., and this treatment was followed byimmersion in a l g./l. chromic acid solution in distilled water, at 70C., also for 15 seconds. The coating thus applied was not visible.

EXAMPLE V Galvanized steel panels were cleaned for 30 seconds by simpleimmersion in alkaline cleaner, rinsed, and treated for 15 seconds in-a1.2 g./l. solution of Armac HT in distilled water at 60 C. The panelswere then divided into two groups. "The first group was immersed in 0.1%chromic acid and the second in 0.1% potassium dichromate (both solutionsmade with distilled water and havinga temperatureof 70' C.).

EXAMPLE VI Four sets of galvanized steel panels, cathodically cleaned inalkali, were treated in solutions of Armac HT ranging in concentrationfrom 0.1% to 1% (15 second immersion, distilled make-up water and 60C.bath temperature), followed by treatment in 0.1% chrornic acid asdescribed. At 0.1% and 0.2% amine salt the films depositedwereundetectable; at 0.5 and 1%. they appeared as .a veryslightcloudiness onthe metalsurface. This cloudiness, it was found in subsequent tests,"could be reduced by thorough rinsing prior to the chromate treatment.

EXAMPLE VII Four sets of galvanized steel panels, cathodically cleanedin alkali, rinsed, and treated in 0.1% Armac HT solution, were thentreated in chromic acid solutions (in distilled water for 15 seconds at70 C.) ranging in concentration from 0.1% to 2%. At 0.1% CrO theresulting film was invisible; at 0.5% the panels were slightly stainedwith yellow streaks; at 1% and 2% this staining became progressivelymore pronounced, and at 1% was too severe to be commercially acceptable.Subsequent rinsing proved to be slightly helpful in reducing this stain,but insuflicient to overcome it adequately.

EXAMPLE VIII Panels of galvanized steel I except that the amine saltwere treated as in Example used was Armac 18, which is esentiallyoctadecylamine acetate, also made by Armour and Company. Corrosion testson these panels compared favorably with those of Example I.

EXAMPLE IX Three groups of galvanized steel panels were treated asdescribed in Example I, with the exception that the second and thirdgroups were immersed in chromic acid solutions that contained 75 p.p.m.sodium sulfate and 75 p.p.m. NaCl respectively. The first group wasbright and unstained; the second group was objectionably discolored, andthe third group exhibited some cloudiness.

The results of corrosion tests, described in Example I, applied to allthe panels prepared in the above examples, are given in the followingtable:

Table Water Film Test Condensation Test Panels Example it No. of Degreeof No. of Degree of Cycles Corrosion Cycles Corrosion IUntreated I IVUntreated 000000000 0 O H- H 1 Scoring: 0=none, =l:=traoe, l+=sllght,2+=moderate, 3+=severe. i Galvanized steel panel, used as control.

1 16 hours instead of 3.

4 82 5 Aluminum alloy panel, used as control.

The process of this invention is treating zinc surfaces (i.e.,principally galvanized iron or steel), wherein it is an exceptionallyeffective, inexpensive and convenient way to prevent the formation ofwhite rust, a disfiguring type of corrosion that plagues themanufacturers of galvanized stock through its strong predilection forstacks and coils of galvanized stock in shipment or storage. It has,however, been found that the advantages conferred on zinc surfaces bythis treatment are also obtainable and often as badly needed on othertypes of metal surfaces, such as those of steel, aluminum, cadmium, etc.

.Having thus described our invention, we claim: .1. The method ofprotecting metal surfaces selected from the class consisting of zinc,cadmium, iron, steel, and aluminum surfaces from corrosion comprisingfirst contacting such surface with a solution of a fatty amine saltwhich is at least water dispersible and which has the formula R-NH Xwhere R is an alkyl group having at least 8 carbon atoms and X is ananion, thereby depositing a hydrophobic fatty amine salt layer on thebare metal, and thereafter bringing said amine salt layer into directcontact with a solution of a hexavalent chromium compound.

2. The method of claim 1 in which the fatty amine salt is in water.

3. The method of claim salt is in aqueous solution.

4. The method of claim 3 in which the said anion X is selected from thegroup consisting of acetate, lactate and chloride.

5. Method of claim 4 in which the fatty amine salt is aqueous solutionat a concentration of 0.01 to 1%. 6. The method of claim 1 wherein saidsolution of salt and said solution of chromium compound contain lessthan 5 p.p.m. of any of the anions chloride, sulfate, phosphate andsilicate.

7. The method of protecting metal surfaces selected from the classconsisting of zinc, cadmium, iron, steel and aluminum surfaces fromcorrosion comprising first contacting said surface with a solutioncontaining a fatty amine salt which is at least water dispersilblehaving the formula RNH X wherein R is an alkyl group having at least 8carbon atoms and X is an anion of the group consisting of acetate,lactate and chloride, said salt being present in said solution in anamount of 0.01 to 1.0%, whereby a hydrophobic fatty amine salt layer isdepositcd on the bare metal, and thereafter bringing said amine saltlayer into direct contact with a solution containing 0.05 to 0.5 of ahexavalent chromium compound, calculated as CrO 8. The method of claim 7wherein said alkyl group contains between 8 and 18 carbon atoms and saidanion is acetate.

9. The method of claim 8 wherein said salt is pres ent in an amount of0.05 to 0.20% and said chromium compound is present in an amount of 0.1to 0.2%.

10. The method of preventing white rust on galvanized stock comprisingcathodically cleaning the stock in alkali, rinsing off the entrainedalkali, applying a 0.01 to 1% solution of a fatty amine salt therebydepositing a hydrophobic fatty amine salt layer on the bare metal, andthen applying a 0.05 to 0.5% solution of a hexavalent chromium compounddirectly to said fatty amine salt layer, said salt having the formulaRNH X wherein R is an alkyl group having at least 8 carbon atoms and Xis an anion of the group consisting of acetate, lactate and chloride.

11. The method of inhibiting corrosion of metal surfaces selected fromthe class consisting of zinc, cadmium, iron, steel and aluminum surfacescomprising cathodically cleaning the metal surface in an alkalinesolution, applying to said surface a solution of a fatty amine salt,thereby to deposit a hydrophobic fatty amine salt layer on the baremetal and then applying directly to said amine particularly. useful in 1in which the fatty amine salt layer a solution of a hexavalent chromiumcompound, said amine salt being at least water dispersible and havingthe formula R-NH X herein R is an alkyl grouphaving 8 to 18 carbon atomsand X is an acetate ion, said salt being present in an amount of 0.01 to1%, ,said chromium compound being present in an amount of 0.05 to 0.5%,calculated as CrO 12. An article having a surface of a metal selectedfrom the class consisting of zinc, cadmium, iron, steel, and aluminumprovided with a corrosion-inhibiting film prepared by deposting ahydrophobic, fatty amine salt layer on the bare metal and thencontacting saidsalt layer directly'with a solution of a hexavalentchromium compound, said .salt being at least Water-dispersible andhavdrocarbon group having at least 8 carbon atoms and X is an anion.

References Cited in the file of this patent UNITED STATES PATENTS2,206,064- Thompson et a1. July 2, 1940 2,400,784 Rust May 21, 19462,460,259 Kahler Ian. 25, 1949 10 2,793,932 Kahler et al. May 28, 1957OTHER REFERENCES Protective Coatings for Metals (Burns and Bradley),published by Reinhold, New York, N.Y., 1955 ing the formula R-NH X,wherein R is an aliphatic hy- 15 (pages 552454).

10. THE METHOD OF PREVENTING WHITE RUST ON GALVANIZED STOCK COMPRISINGCATHODICALLY CLEANING THE STOCK IN ALKALI, RINSING OFF THE ENTRAINEDALKALI, APPLYING A 0.01 TO 1% SOLUTION OF A FATTY AMINE SALT THEREBYDEPOSITING A HYDROPHOBIC FATTY AMINE SALT LAYER ON THE BARE META, ANDTHEN APPLYING A 0.05 TO 0.5% SOLUTION OF A HEXAVALENT CHROMIUM COMPOUNDDIRECTLY TO SAID FATTY AMINE SALT LAYER, SALT HAVING THE FORMULAR-NH3XWHEREIN R IS AN ALKYL GROUP HAVING AT LEAST 8 CARBON ATOMS AND X IS ANANION OF THE GROUP CONSISTING OF ACETATE, LACTATE AND CHLORIDE.